1. Field of the Invention
The present invention relates to an electromagnetic bandgap structure, more specifically to an electromagnetic bandgap structure and a printed circuit board having the same that prevent a signal ranging a predetermined frequency band from being transmitted.
2. Background Art
New electronic apparatuses and communication apparatuses are increasingly becoming smaller, thinner and lighter, reflecting today's emphasis on growing mobility.
These electronic and communication apparatuses have various complex electronic circuits (i.e. analog circuits and digital circuits) for performing their functions and operations. These electronic circuits typically carry out their functions by being implemented in a printed circuit board (PCB). The electronic circuits on the PCB commonly have different operation frequencies from one another.
The printed circuit board in which various electronic circuit boards are implemented often has a noise problem, caused by the transfer and interference of an electromagnetic (EM) wave resulted from the operation frequency and its corresponding harmonics components of one electronic circuit to another electronic circuit. The transferred noise can be roughly classified into radiation noise and conduction noise.
The radiation noise (refer to the reference numeral 155 of FIG. 1) can be easily prevented by covering a protective cap on the electronic circuit. However, preventing the conduction noise (refer to the reference numeral 150 of FIG. 1) is not as easy, because the conduction noise is transferred through a signal transfer path inside the board.
The noise problem will be described in more detail with reference to FIG. 1. FIG. 1 is a sectional view showing a printed circuit board including two electronic circuits having different operation frequencies. Although FIG. 1 shows a 4-layered printed circuit board 100, it shall be obvious that the printed circuit board can be modified to have a 2, 6 or 8-layered structure.
As shown in FIG. 1, the printed circuit board 100 includes metal layers 110-1, 110-2, 110-3 and 110-4 (hereinafter, collectively referred to as 110) and dielectric layers 120-1, 120-2 and 120-3 (hereinafter, collectively referred to as 120) interposed between metal layers 110. The top metal layer 110-1 of the printed circuit board 100 is implemented with two electronic circuits 130 and 140 having different operation frequencies (hereinafter, referred to as a first electronic circuit 130 and a second electronic circuit 140, respectively). Here, both of the first electronic circuit 130 and the second electronic circuit 140 are assumed to be digital circuits.
Here, if it is assumed that the metal layer represented by the reference numeral 110-2 is a ground layer and the metal layer represented by the reference numeral 110-3 is a power layer, each ground pin of the first electronic circuit 130 and the second electronic circuit 140 is electrically connected to the metal layer represented by the reference numeral 110-2 and each power pin is electrically connected to the metal layer represented by the reference numeral 110-3. In the printed circuit board 100, every ground layer is also electrically connected to each other through vias. Similarly, every power layer is also electrically connected to each other through vias (refer to the reference numeral 160 of FIG. 1).
If the first electronic circuit 130 and the second electronic circuit 140 have different operation frequencies, a conductive noise 150 caused by an operation frequency of the first electronic circuit 130 and its harmonics components is transferred to the second electronic circuit 140 as shown in FIG. 1. This has a disadvantageous effect on the accurate function/operation of the second electronic circuit 140.
With the growing complexity of electronic apparatuses and higher operation frequencies of digital circuits, it is increasingly more difficult to solve this conduction noise problem. Especially, the typical bypass capacitor method or decoupling capacitor method for solving the conductive noise problem is no longer adequate, as the electronic apparatuses use a higher frequency band.
Moreover, the aforementioned solutions are not adequate when several active devices and passive devices need to be implemented in a complex wiring board having various types of electronic circuits formed on the same board or in a narrow area such as a system in package (SiP) or when a high frequency band is required for the operation frequency, as in a network board.
Accordingly, an electromagnetic bandgap structure (EBG) has currently come into the spotlight to solve the foresaid conductive noise problem. This aims to block a signal of a predetermined frequency band by disposing an electromagnetic bandgap structure having a predetermined structural shape in a printed circuit board.
An electromagnetic bandgap structure having a mushroom type structure as shown in FIG. 2A and FIG. 2B has been studied.
The electromagnetic bandgap structure 200 shown in FIG. 2A and FIG. 2B is formed by repeatedly arranging a mushroom type structure 230, which includes a metal plate 232 formed between the first metal layer 211 and the second metal layer 212 and a via 234 connecting the first metal layer 211 to the metal plate 232, between a first metal layer 211 and a second metal layer 212, each of which functions as a ground layer and a power layer. A first dielectric layer is interposed between the first metal layer 211 and the metal plate 232, and a second dielectric layer is interposed between the metal plate 232 and the second metal layer 212.
As such, arranging the mushroom type structure 230 between the first metal layer 211 and the second metal layer 212 repeatedly can allow a signal x of a low frequency band (refer to FIG. 2C and FIG. 2D) and a signal y of a high frequency band (refer to FIG. 2C and FIG. 2D) to pass through the electromagnetic bandgap structure 200 and block a signal z of a certain frequency band (refer to FIG. 2C and FIG. 2D) ranging between the low frequency band and the high frequency band. In other words, the electromagnetic bandgap structure 200 shown in FIG. 2A and FIG. 2B can function as a band stop filter blocking a signal of a certain frequency band. This can be easily understood through an equivalent circuit of FIG. 2C.
In the equivalent circuit of the mushroom type electromagnetic bandgap structure 200 shown in FIG. 2C, a capacitance component C1 and an inductance component L1 are connected in series between the first metal layer 211 and the second metal layer 212. Here, C1 is a capacitance component formed by the second metal layer 212, the second dielectric layer 222 and the metal plate 232, and L1 is an inductance component formed by the via 234 placed between the metal plate 232 and the first dielectric layer 221. As a result, the mushroom type bandgap structure 200 can function as a kind of band stop filter by this L-C serial connection.
However, it is difficult to apply the mushroom type electromagnetic bandgap structure 200 in various apparatuses because the mushroom type electromagnetic bandgap structure 200 functions as a band stop filter by using one inductance component and one capacitance component only. This is because the acquirable length, (i.e. corresponding to an inductance value) of the via 234 is limited in the structural shape of FIG. 2A and FIG. 2B. The acquirable capacitance value is also limited because the mushroom type structure 230 is placed between two adjacent metal layers only.
As new electronic apparatuses and communication apparatuses are increasingly becoming smaller, thinner and lighter, it is more difficult to select a desired bandgap frequency band by using the mushroom type electromagnetic bandgap structure 200 only. In other words, the mushroom type electromagnetic bandgap structure 200 shown in FIG. 2A and FIG. 2B has some restrictions in adjusting each bandgap frequency band to meet conditions and features of various application apparatuses or to lower a conductive noise to a desired noise level in a pertinent bandgap frequency band.
Accordingly, it is necessarily required to study the structure of the electromagnetic bandgap that not only can outstandingly block or reduce a conductive noise between a power layer and a ground layer but also can be universally applied to various application apparatuses having different bandgap frequency bands.